Prep the pan. Heat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease.

Melt the butter. In a large bowl, melt butter until smooth. Let it cool 2–3 minutes so it doesn’t scramble the eggs.

Whisk in sugars. Add granulated and brown sugar to the butter and whisk until glossy and combined, about 30 seconds.

Add wet ingredients. Whisk in Greek yogurt, eggs, and vanilla until the mixture looks smooth and slightly thick.

Fold in dry ingredients. Sift in cocoa powder, flour, salt, and espresso powder (if using). Switch to a spatula and fold gently until just combined. Do not overmix.

Add chocolate. Fold in chocolate chips. The batter will be thick and shiny.

Spread and smooth. Scrape batter into the pan and smooth the top. Sprinkle with flaky sea salt if you like a sweet-salty finish.

Bake. Bake 22–26 minutes, until the edges are set and a toothpick in the center comes out with moist crumbs, not wet batter. For extra fudgy, aim closer to 22–24 minutes.

Cool completely. Let brownies cool in the pan on a rack for at least 1 hour. They firm up as they cool, making cleaner slices.

Slice and serve. Lift out using parchment and cut into 9 large or 16 small squares. Enjoy as is or warm slightly for gooey centers.